Non-destructive Ion Beam Analysis of Art Objects

Aim

Preservation

Early detection of incipient deterioration or rather the diagnosis of corresponding potential risks ensures preventive conservation of endangered art objects made from glass of non-resistant composition.

Art technology

Getting knowledge on the particular technology and the materials used for the creation of special aesthetic effects helps art scientists to identify individual artists from their artistic works. Moreover, restorers deduce the demand of precautionary activities, e.g. to prevent alteration of colours or gradual deterioration of sensitive backing materials like papers and parchments.

Authenticity

In certain cases elemental analysis reveals copies or even forgeries. In particular this becomes possible when detecting characteristic elements of pigments which came up in a later period than the object was ascribed to.

Method

The focused beam of accelerated protons leaves the vacuum tube through a thin HAVAR® exit window and strikes the object which is positioned on air. Along the proton path in matter Coulomb interactions and nuclear reactions take place with atoms of the material under analysis. The hit atoms emit X-radiation and Gamma-radiation the energy of which is characteristic for the particular chemical element. The arrangement of X-ray and Gamma-ray detectors allows multi-element analysis (PIXE and PIGE) within nearly the whole periodic system. Simultaneous detection of backscattered protons (RBS) highlights elements on the object surface and provides depth information in the near-surface region of the irradiated position.

Objects

Silica based materials

i.e. museum objects made from ancient glass or enamel. In dependency of its composition glass gets affected by the environmental humidity. As a consequence glass surfaces may deteriorate. Degraded glass can not be restored. Thus, early warning, special storage conditions and preventive conservation are essential. Surfaces of glass objects may have been modified for obtaining special visual effects, e.g. iridescence. Such technologies can be explored using PIXE in combination with RBS.

Painting

Pigments of oil paintings and pastel drawings can be deduced from PIXE spectra; sensitive water-colour paintings, book painting, hand drawings and inks on paper or parchment can be examined non-destructively as well. The method is even useful for studying painting on glass and porcelain surfaces.

Metals

Patina layers, e.g. on coins of copper/silver alloys, may extend to a thickness of more than hundred microns which is above the information depth of ion beam analysis. Gold is the most non-reactive of all metals. Gold tarnish is very thin and shows up as a darkening of reflecting surfaces. Other noble metals are quite resistant to corrosion, although silver readily forms a surface tarnish of silver sulphide. Thus, only gold can be analyzed without any mechanical surface treatment or cutting the metal for analyzing the cross section.

Organic lakes and coverings

Their presence on surfaces, e.g. varnish on a painting, can be detected using RBS. Foreign elements in organic materials are detectable using PIXE/PIGE.

Material

Analytical Information

Glass, Enamel

tate of preservation

Painting

characteristic elements of pigments, secondary and trace elements within

Limits

Territorial fixing of the analysis to the location of the particle accelerator

Chemical bonds can not be cleared up

Organic coverings can be indicated but not chemically identified

Preselection

Risks of transport, the interrupt of air-conditioning, costs for packing and assurance etc. demand a preselection of art objects. This should be carefully accomplished on-site, i.e. already in the museum, before accelerator based analytical work. On-site testing is ideal by making use of a mobile mini XRF facility, not larger than a common TV camera.

Examples

Fig. 2: Comparison of RBS spectra from intact and leached glass: Glass corrosion is indicated by both missing Ca atoms at the glass surface (shift of the Ca high energy edge) and the enrichment of oxygen at the surface (O surface peak).

Paint layers

Identification of paint layers is possible by variation of the primary proton energy. PIXE spectra gained at different proton energies Ep give relative concentrations of pigment related elements Ze at varied information depths. With increasing Ep a progressive increase/decrease of the concentration of Ze indicates the presence of this element within a deeper/upper layer of the pigment arrangement. Using this technique, fundamental experience was got from PIXE studies at test paint layer arrangements.

PIXE results are obtained as a function of the incident proton energy Ep.

Oil painting

The primary painting technique of Lucas Cranach the Elder was studied examining his early work “14 Nothelfer” using PIXE at two different proton energies 2.1 MeV and 3.9 MeV, respectively. As an example, the results are given for the red garment of the Holy Christopherus.

Ep[MeV]

Hg

Pb

Ca

2.1

56.9

32.1

8.7

3.9

35.9

57.1

4.9

Fig. 7: Relative concentrations* (wt%) of main and secondary elements visible in the PIXE spectra of the red garment of the Holy Christopherus (“14 Nothelfer”, Lucas Cranach the Elder).

Fig. 6: X-ray spectra from the red garment of the Holy Christopherus (“14 Nothelfer”, Lucas Cranach the Elder) taken at two proton energies 2.1 MeV and 3.9 MeV, respectively.

The table identifies a paint layer arrangement: White lead (Pb) under Cinnabar (Hg, S), producing additively the visually lightened red of the garment.

The 2.1 MeV proton beam interacts mainly with atoms of the cinnabar (HgS) top layer. With increasing proton energy, i.e. at 3.9 MeV in this case, the Pb atoms arranged in a paint layer underneath gradually contribute to the X-ray spectrum.

The Ca X-rays originate from chalk priming but may also come from constituents of the painting materials. Low energy Ca-K X-radiation (ECa-K = 3.7 keV) gets strongly absorbed by Hg and Pb atoms. Therefore, the Ca-K X-ray intensities are not used for interpretations.

Final remarks

Evaluation of the PIXE spectrum allows obtaining characteristic main elements as well as secondary elements and impurities. Pigments can be deduced by experts from their knowledge on painting technologies. Whereas the pigment related elements characterize the historic époque of creation the secondary elements may help to clarify the provenance of the painting. The PIGE spectrum represents a valuable completion regarding light elements (Z<15). The presence of organic overlayers, like varnish or organic lake, is indicated by the RBS spectrum. Chemical bonds cannot be characterized by these methods.

The depth arrangement of characteristic elements, i.e. the arrangement of paint materials in kind of layers or as admixed pigments, assists to clarify the painting technique of the individual artist. PIXE at different proton energies, hence varied information depth, is used to solve this problem.

Gold

The “Sky Disk of Nebra” (Bronze Age), one of the most spectacular archaeological discoveries in recent years was found in Saxony-Anhalt (Sangerhausen, Germany, 1997/98) and it was brought to the attention of the German public in 2002. For each detail on the disk the gold composition was measured in order to clarify whether it belongs to the original version or possibly to a later additive.

Fig. 8: The “Sky Disk of Nebra” positioned for gold analysis at the external proton beam of the Research Centre Dresden-Rossendorf (left side) together with a typical PIXE spectrum (right side). The Ag/Au and Cu/Au concentration ratios have been used for characterization of the particular gold applications.

Abb. 9: Christian Neelmeijer at the external ion beam

The “Sky Disk of Nebra” is part of a larger hoard discovery containing amongst ancient jewellery two swords of typical Bronze Age. Gold rings ornamenting their handles show compositions comparable with that of the stars, sun, moon and horizon fit on the Sky Disk.